The background of the invention will be discussed in two parts:
1. Field of the Invention
This invention relates to the welding of metals and plastics with a laser.
2. Description of Prior Art
The welding of metals with a laser is accomplished by focusing a high power laser beam on or near the metal. The usual power density achieved at the focus is in the range of 1,000,000 watts per square centimeter. This high power density is capable of forming a hole in the metal usually referred to as a "key hole." The key hole is surrounded by molten metal. After the key hole is formed, the focus laser spot is slowly translated relative to the metal. This causes the key hole to move. Molten metal flows around both sides of the key hole and comes together at the trailing edge of the key hole. The heat capacity in the remainder of the metal which was not raised to the liquid state rapidly cools the molten metal solidifying the weld as the key hole is translated through the metal.
The objective in welding is to attach two pieces of metal with the weld. One type of weld is referred to as a "butt joint." In this type of weld, two pieces of metal are joined together edge to edge. In making a butt joint, the prior art laser welding technique has the focused laser beam striking the seam between two pieces of metal. The other common laser weld is for the two pieces of metal to be over lapped so that the laser beam must penetrate through one layer of metal in order to reach the second piece of metal. Often, the key hole passes through both pieces of metal. This type of weld is commonly referred to as a "lap joint." For laser welding, achieving a good lap joint is easier and more reliable than a good butt joint. One reason for this is because the sheets of metal can be easily deformed in the thickness dimension to minimize the size of the gap in a lap weld. However, the metal is much stiffer in the plane of the metal sheet; therefore, butt welds have more problems with gaps. In a butt weld, the laser beam is striking the exact location where the two pieces of metal form the seam. Under ideal conditions, these two pieces of metal are contacting everywhere along the seam. In practice, tolerances and fixturing can cause gaps to appear in the seam. The deposition of heat into the two pieces of metal relies on the absorption of the laser beam by the metal. Gaps in the seam can affect the amount of laser power which is converted to heat in the metal. Also, gaps can affect the ability of molten metal to flow together in the wake of the key hole passage. Imperfect closure behind the key hole will cause a poor weld.
The present invention describes a process whereby the absorption of laser power in the vicinity of the seam of a butt joint is controlled more reliably even when there are slight gaps in the seam. Also, the process provides a more positive closure of the seam by the molten metal thereby forming a more reliable weld relative to prior art. Also, lap joints can be made at an improved speed under certain conditions compared to prior art.